Guide wire and stent

ABSTRACT

A guide wire includes a distal core member made of a ferrous alloy which has shape memory properties and superelasticity. The ferrous alloy preferably includes substantially two phases, and has a difference of 100° C. or less between an Af point and an Ms point in a thermal hysteresis of martensitic transformation and reverse transformation. The guide wire may include a proximal core member made of an iron-containing alloy and having a higher modulus of elasticity than the distal core member. The two core members may be joined together by welding to form a core of the guide wire.

CROSS REFERENCE TO RELATED

This application is a division of pending application Ser. No.12/108,882 filed Apr. 24, 2008, the contents of which are incorporatedherein by reference, which claims priority under 35 U.S.C. §119 toJapanese Application No. 2007-124871 filed May 9, 2007, the entirecontents of which are hereby incorporated by reference.

BACKGROUND

The present invention relates to a medical device. More particularly,the invention relates to a guide wire and stent for insertion into abody lumen such as a blood vessel or a bile duct.

One technique that is used in the diagnosis and treatment of conditionssuch as heart disease involves inserting a guide wire to a target site,then inserting and passing a catheter or the like along the guide wire.

For example, in percutaneous coronary intervention (PCI), first a guidewire is advanced to, then made to traverse, the area of stenosis that isthe target site while selecting the branches of the coronary arteryunder fluoroscopic guidance. Next, a dilatation catheter fitted at thedistal end with a balloon is introduced into the body along the guidewire, and the balloon on the catheter is positioned at the stenosis. Bythen expanding the balloon and enlarging the lumen at the stenosis,blood flow is ensured. Conditions such as angina pectoris can be treatedin this way.

Treatment can also be carried out by introducing a self-expanding stentto the target stenosis, and allowing the stent to self-expand at thestenosis.

Titanium-nickel alloys are sometimes used as the material making up suchguide wires and stents. When a titanium-nickel alloy is used as theguide wire core, because the pushability and torque transmission of suchalloys are inferior to those of materials such as stainless steel, thesurface is sometimes coated with a resin to ensure good slideability.

To enhance the torque transmission and other properties, JP 3-264073 Adiscloses a catheter guide wire core that is a linear body made of aferrous superelastic metal material, the core being distally tapered soas to become increasingly flexible toward the distal end. Examples offerrous superelastic metal materials include Fe—Pt, Fe—Pd, Fe—Ni—Co—Ti,Fe—Ni—C, Fe—Mn—Si, Fe—Cr—Mn, Fe—Cr—Mn—Si, Fe—Cr—Ni—Mn—Si andFe—Cr—Ni—Mn—Si—Co alloys. These are noted as being desirable becausethey have a high elasticity and are not readily prone to plasticdeformation.

In connection with such ferrous superelastic metal materials, JP03-257141 A, JP 2003-268501 A, JP 2000-17395 A, JP 09-176729 A, andScripta Materialia 46, 471-475 describe Fe—Ni—Co—Al—C alloys, Fe—Ni—Alalloys, Fe—Ni—Si alloys, Fe—Mn—Si alloys and Fe—Pd alloys.

Good superelasticity is not achieved in conventional ferrous alloysbecause of, at the time of deformation, (a) the introduction ofpermanent strain such as dislocations, and (b) the formation ofirreversible stress-induced lenticular martensite which does not exhibitshape memory effects. To avoid problems (a) and (b), it is effective toimprove the strength of the matrix phase in ferrous shape-memory alloys;materials which are precipitation strengthened with an intermetalliccompound are particularly effective. It is considerations such as thesethat have led to the disclosure of the foregoing ferrous alloys.

Although Ti—Ni alloys such as the above are sometimes used in guidewires and stents, the superelastic strain region for Ti—Ni alloys is atbest about 8%; when a larger deformation is applied, the alloy undergoesplastic deformation, which is undesirable. It would be preferable to usea material having a broader superelastic strain region than Ti—Ni alloysas the guide wire and stent material.

In cases where a Ti—Ni alloy is used as the core material on the distalportion of a guide wire, a ferrous alloy is used as the core material onthe proximal portion, and these core materials are welded together toform a single guide wire core, the fact that the Ti—Ni alloy is adifferent material which has, in particular, a poor weldability withferrous alloys, imposes limitations on the materials to which it can bebonded and the bonding conditions. In medical devices for insertion andextended placement within the body in particular, the device must bemanufactured while taking the greatest possible care to avoid thepossibility of the weld failing within the body; hence the need forspecial bonding conditions, etc.

As noted above, in some cases where a Ti—Ni alloy is used as the guidewire core, the surface is coated with a resin. However, when a plasticmaterial having a high melting point, such as a fluorocarbon resin, isused as the resin, the properties of the Ti—Ni alloy may change underthe influence of the high temperature. The distal portion of the guidewire core, while slender, undergoes repeated bending and twisting. To beable to withstand such use, the resin coated onto the surface must havean improved peel resistance.

In addition, stents made of Ti—Ni alloys are sometimes of insufficientstrength and durability. It is especially difficult to satisfy therequirements for strength and durability in stents used at placementsites where there is a lot of movement, such as the legs. Also, while itis preferable for a stent to be thin-walled, the strength in such casesdecreases even further. Hence, thin-walled Ti—Ni alloy stents are oftenunable to withstand normal use.

Guide wires and stents made of Ti—Ni alloys also lack a good visibilityunder fluoroscopic imaging. To enable the insertion site and placementsite to be verified under fluoroscopic imaging, a high-contrast membermade of gold or the like must be bonded to the distal end of the guidewire and the ends of the stent.

The ferrous metal materials listed in JP 3-264073 A as preferable foruse in guide wires are referred to as being “superelastic,” yet theamount of strain from which superelastic recovery is possible in suchmaterials is in fact less than 1%, which hardly satisfies the propertiesrequired of a guide wire core.

Nor do JP 03-257141 A, JP 2003-268501 A and JP 2000-17395 A make anymention of such properties of practical importance as the amount ofstrain from which superelastic recovery is possible, the percentrecovery, and the superelastic operating temperature ranges forFe—Ni—Co—Al—C alloys, Fe—Ni—Al alloys and Fe—Ni—Si alloys.

Scripta Materialia 46, 471-475 reports on the superelasticity of Fe—Pdalloys containing unusually large amounts of costly palladium, but suchalloys have an amount of strain from which superelastic recovery ispossible of less than 1%, which is small, and thus cannot be regarded ashaving a good superelasticity. Moreover, these alloys are difficult toproduce.

JP 09-176729 A mentions that Fe—Mn—Si alloys are nonmagnetic but, byutilizing a fcc/hcp transformation, exhibit a shape memory effect andsuperelasticity. However, there are limitations on the temperatures atwhich such Fe—Mn—Si alloys can be used because the superelasticity isachieved at temperatures higher than room temperature. Moreover, thesealloys have a poor corrosion resistance and cold workability,complicated thermomechanical treatment is required to achieve furthersuperelasticity, and there are problems with the manufacturability.

SUMMARY

A guide wire includes a member having a ferrous alloy. According to oneaspect, the ferrous alloy may have shape memory properties andsuperelasticity, include substantially two phases having a γ phase and aγ′ phase, and have a difference of 100° C. or less between a reversetransformation finish temperature and a martensitic transformation starttemperature in a thermal hysteresis of martensitic transformation andreverse transformation. The guide wire also includes a proximal coremember which is made of an iron-containing alloy and has a highermodulus of elasticity than the distal core member. The distal coremember and the proximal core member are joined together by welding toform a core of the guide wire. The proximal core member preferably ismade of stainless steel or is piano wire. The guide wire may include atubular member which covers the distal core member. The tubular memberpreferably is a coil. The tubular member preferably is made of plastic.

According to another aspect, a guide wire may includes a proximal tubewhich may be made of a metallic material having a higher modulus ofelasticity than the ferrous alloy and which covers at least part of theproximal portion of the core, the ferrous alloy has shape memoryproperties and superelasticity, includes substantially two phases havinga γ phase and a γ′ phase, and has a difference of 100° C. or lessbetween a reverse transformation finish temperature and a martensitictransformation start temperature in a thermal hysteresis of martensitictransformation and reverse transformation. The guide wire may preferablyfurther include a tubular member which covers a distal portion. Thetubular member may be made of plastic.

According to further aspect, a guide wire may include a coating whichhas at least one layer and covers at least part of a surface of theproximal core member, the at least one layer of the coating is made of afluorocarbon resin, and the ferrous alloy has shape memory propertiesand superelasticity, includes substantially two phases having a γ phaseand a γ′ phase, and has a difference of 100° C. or less between areverse transformation finish temperature and a martensitictransformation start temperature in a thermal hysteresis of martensitictransformation and reverse transformation.

A stent includes a body made of a ferrous alloy. According to anotheraspect, the ferrous alloy preferably has shape memory properties andsuperelasticity, includes substantially two phases having a γ phase anda γ′ phase, and has a difference of 100° C. or less between a reversetransformation finish temperature and a martensitic transformation starttemperature in a thermal hysteresis of martensitic transformation andreverse transformation. The body preferably has a plurality ofundulating bends provided in an axial direction. The body is preferablycomposed of braided wire.

According to furthermore aspect, a medical device includes a firstmember including a ferrous alloy. A second member may be joined with thefirst member. A member, for example a wire, a plate, a pipe or a rod, ismade of a ferrous alloy having shape memory properties andsuperelasticity, including substantially two phases having a γ phase anda γ′ phase, and having a difference of 100° C. or less between a reversetransformation finish temperature and a martensitic transformation starttemperature in a thermal hysteresis of martensitic transformation andreverse transformation, the ferrous alloy including a composition whichincludes from 25 to 35% by mass of nickel, from 10 to 30% by mass ofcobalt, and from 2 to 8% by mass of aluminum, the composition includinga total of from 1 to 20% by mass of at least one selected from the groupconsisting of from 1 to 5% by mass of titanium, from 2 to 10% by mass ofniobium, from 3 to 20% by mass of tantalum, and the balance being from35 to 50% by mass of iron and inadvertent impurities. An abundance of aspecific crystal orientation in a working direction for the ferrousalloy preferably is at least 2.0. The medical device may be a guidewire.

The guide wire of an embodiment according to a first aspect (alsoreferred to below as “guide wire (A)”) includes a core having a distalcore member made of a ferrous alloy (also referred to below as “theferrous alloy of the invention”) which has shape memory properties andsuperelasticity, consists substantially of two phases having a γ phaseand a γ′ phase, and has a difference of 100° C. or less between areverse transformation finish temperature and a martensitictransformation start temperature in a thermal hysteresis of martensitictransformation and reverse transformation, and a proximal core memberwhich is made of an iron-containing alloy and has a higher modulus ofelasticity than the distal core member, the distal core member andproximal core member being joined together by welding. The guide wirehas a broad superelasticity region and a high Young's modulus, and isthus endowed with both suppleness and pushability. Moreover, the guidewire has an excellent weldability between the distal core member and theproximal core member. In addition, when coated on the surface with ahigh-melting plastic material such as a fluorocarbon resin, this guidewire is not readily affected by heat treatment and has an excellentslideability. This guide wire also has an excellent fluoroscopicvisibility. Furthermore, it uses a superelastic ferrous alloy that isrelatively easy to produce. In addition, this guide wire has a smallerLPS (recovery stress)—a tensile property—than Ti—Ni alloys, and is thusmore gentle on blood vessels. Finally, because the maximum tensilestrength is higher than that of Ti—Ni alloys, this guide wire has a highsafety even after it has been tapered.

The guide wire of an embodiment according to a second aspect (alsoreferred to below as “guide wire (B)”) includes a core having distal andproximal portions, of which at least the distal portion is made of theferrous alloy of the invention, and a proximal tube which is made of ametallic material having a higher modulus of elasticity than the ferrousalloy making up the core and which covers at least part of the proximalportion of the core. This guide wire (B), owing to a broadsuperelasticity region and a high Young's modulus, is endowed with bothsuppleness and pushability. Moreover, when coated on the surface with ahigh-melting plastic material such as a fluorocarbon resin, it is notreadily affected by heat treatment and has an excellent slideability. Inaddition, this guide wire has an excellent bondability between thedistal core and the proximal tube. This guide wire also has an excellentfluoroscopic visibility. Furthermore, it is made using a superelasticferrous alloy that is relatively easy to produce.

The guide wire of an embodiment according to a third aspect (alsoreferred to below as “guide wire (C)”) includes a distal core member anda proximal core member, each made of a ferrous alloy, and a coatingwhich is composed of at least one layer and covers at least part of asurface of the proximal core member, at least one layer of the coatingbeing made of a fluorocarbon resin. This guide wire (C), owing to abroad superelasticity region and a high Young's modulus, is endowed withboth suppleness and pushability. Moreover, it is a guide wire which,when coated on the surface with a high-melting plastic material such asa fluorocarbon resin, is not readily affected by heat treatment and hasan excellent slideability. In addition, it has an excellent fluoroscopicvisibility.

The stent of an embodiment according to a fourth aspect, which includesa body composed of a ferrous alloy, is endowed with excellent strengthand durability. The stent has excellent fluoroscopic visibility.Moreover, the body is composed of a superelastic ferrous alloy that isrelatively easy to produce.

The invention can provide such guide wires and stents.

BRIEF DESCRIPTION OF THE DIAGRAMS

In the accompanying drawings:

FIG. 1 is a longitudinal sectional view showing a first embodiment of aguide wire (A) according to one aspect of the present invention;

FIG. 2 presents diagrams illustrating a procedure for connectingtogether the distal core member and the proximal core member in theguide wire (A) shown in FIG. 1;

FIG. 3 is a longitudinal sectional view showing a second embodiment of aguide wire (A);

FIG. 4 is a longitudinal sectional view showing a first embodiment of aguide wire (B) according to another aspect of the invention;

FIG. 5 is a longitudinal sectional view showing an embodiment of a guidewire (C) according to yet another aspect of the invention;

FIG. 6 is a front view of a stent according to Examples of theinvention;

FIG. 7 is a development view of the stent shown in FIG. 6;

FIG. 8 is a development view of the stent shown in FIG. 6, in thecontracted state;

FIG. 9 is a partial, enlarged view of the stent shown in FIG. 6;

FIG. 10 shows a typical electrical resistance curve for a shape-memoryalloy (the martensitic transformation start temperature (Ms point) andthe reverse transformation finish temperature (Af point) can bedetermined from an electrical resistance curve of the martensitictransformation in cooling and the reverse transformation in heating);

FIGS. 11A-11E are schematic diagrams showing examples of the sequence ofoperations from first annealing to aging in the production of ferrousalloys in Examples 1 to 5 of the invention and Comparative Example 1(FIG. 11A), and in Examples 6 to 9 of the invention (FIGS. 11B to 11E);

FIG. 12 is a stress-strain curve which is obtained in a tension cyclingtest on a plate at room temperature, and from which the superelasticstrain, Young's modulus and strength (0.2% yield strength) can bedetermined;

FIG. 13 is a stress-strain correlation diagram at an applied strain of2% for the ferrous alloy plate of Example 3;

FIGS. 14A and 14B show inverse pole figures which indicate as contourlines the abundance of γ phase crystal orientations in the rollingdirection of ferrous alloy plates obtained in Example 6 (FIG. 14A) andExample 9 (FIG. 14B);

FIG. 15 is a stress-strain correlation diagram at an applied strain of15% for the ferrous alloy plate of Example 9;

FIG. 16 is a perspective view of a vascular filter composed of theferrous alloy of the invention; and

FIG. 17 is a perspective view of an orthodontic wire composed of theferrous alloy of the invention.

DETAILED DESCRIPTION

First, the guide wire (A) of a first embodiment according to one aspectof the invention is described in detail based on the preferredembodiments shown in the attached diagrams.

FIG. 1 is a longitudinal sectional view showing a first embodiment of aguide wire (A) according to one aspect of the invention; FIG. 2 presentsdiagrams illustrating a procedure for connecting together the distalcore member and the proximal core member in the guide wire (A) shown inFIG. 1; and FIG. 3 is a longitudinal sectional view showing a secondembodiment of a guide wire (A) according to the aspect of the invention.For the sake of convenience, in FIGS. 1 and 2, the portion of the guidewire appearing on the right side of the diagram is referred to herein asthe “proximal side” and the portion of the guide wire appearing on theleft side of the diagram is referred to as the “distal side.” Moreover,in FIGS. 1 and 2, the guide wire is shown schematically so as to appearshortened in the length direction and exaggerated in the thicknessdirection, as a result of which the ratio between the length directionand the thickness direction as shown in the diagrams differsconsiderably from the actual ratio (the same applies also to thesubsequently described diagram in FIG. 3).

A guide wire 1 shown in FIG. 1 is a catheter guide wire for use byinsertion into a catheter. The guide wire 1 has a core 10 obtained bywelding and thereby joining together a distal core member 2 disposed onthe distal side and a proximal core member 3 disposed on the proximalside of the distal core member 2, and also has a helical coil 4. Theguide wire 1 has a total length which, while not subject to anyparticular limitation, is preferably in a range of from about 200 mm toabout 5,000 mm. The core 10 has an outer diameter (referring to theouter diameter of that portion of the core 10 where the outer diameteris constant) which, while not subject to any particular limitation, ispreferably in a range of from about 0.2 mm to about 1.2 mm.

The distal core member 2 is a wire having elasticity which is composedof the ferrous alloy of the invention. The distal core member 2 has alength which, while not subject to any particular limitation, ispreferably in a range of from about 20 mm to about 1,000 mm.

In the present embodiment, the distal core member 2 has a constantdiameter over a given length from the proximal end thereof, thengradually tapers toward the distal direction starting at someintermediate point. This latter portion is referred to herein as thetapering diameter portion 15. By having such a tapering diameter portion15, the rigidity (flexural rigidity, torsional rigidity) of the distalcore member 2 can be gradually reduced in the distal direction. As aresult, the guide wire 1 achieves a good flexibility in the distalportion, thus improving the safety and the ability of the guide wire 1to navigate a blood vessel, and preventing undesirable effects such askinking.

In the illustrated arrangement, the tapering diameter portion 15 isformed over part of the distal core member 2, although it is alsopossible for the tapering diameter portion 15 to make up the entiredistal core member 2. The angle of taper (rate of reduction in the outerdiameter) on the tapering diameter portion 15 may be constant along thelength of the wire or there may be points along the length where theangle of taper changes. For example, a plurality of places where theangle of taper is relatively large and a plurality of places where theangle of taper is relatively small may be repeatedly formed inalternation.

Or the distal core member 2 may have a portion with an outer diameterthat is constant in the lengthwise direction, either partway along thetapering diameter portion 15 or distal to the tapering diameter portion15. For example, the distal core member 2 may have formed, at aplurality of places in the lengthwise direction, tapered portions onwhich the outer diameter tapers in the distal direction, and may haveformed, between one such tapered portion and another such taperedportion, a portion having a constant outer diameter in the lengthwisedirection. The effects achieved in such cases are the same as thoseindicated above.

Alternatively, unlike the arrangement shown in the foregoing diagrams,it is possible for the guide wire 1 to have a configuration in which theproximal end of the tapering diameter portion 15 is situated partwayalong the proximal core member 3; i.e., the tapering diameter portion 15is formed so as to straddle the interface (weld 14) between the distalcore member 2 and the proximal core member 3.

The material making up the distal core member 2 is the ferrous alloy ofthe invention, which is described more fully later in the specification.

Compared with stainless steel, the ferrous alloy of the invention isflexible, in addition to which it has resilience and is not readilysubject to deforming plastically. Therefore, by having the distal coremember 2 made of the ferrous alloy of the invention, the guide wire 1 isable to achieve in the distal portion thereof both sufficientflexibility and sufficient resilience to bending, enhancing the abilityof the guide wire to navigate highly tortuous vasculature, and thusproviding an even better operability. Moreover, even when the distalcore member 2 is repeatedly curved and bent, because of its resilience,the distal core member 2 does not deform plastically, making it possibleto prevent a decline in operability during use of the guide wire 1 dueto plastical deforming of the distal core member 2. Furthermore, becausethe ferrous alloy of the invention has a higher Young's modulus thanTi—Ni alloys, the distal core member 2 retains a suppleness even whenthe diameter is reduced, and thus exhibits the above-describedoperability.

The distal end of the proximal core member 3 is joined to the proximalend of the distal core member 2 by welding. The proximal core member 3is in the form of a wire having elasticity. The length of the proximalcore member 3, while not subject to any particular limitation, ispreferably from about 20 mm to about 4,800 mm.

The proximal core member 3 is made of a material having larger moduli ofelasticity (Young's modulus (longitudinal elastic modulus), modulus ofrigidity (modulus of transverse elasticity), volumetric elastic modulus)than the material making up the distal core member 2. In this way, asuitable rigidity (flexural rigidity, torsional rigidity) can beachieved in the proximal core member 3, giving the guide wire 1 a goodstiffness that enhances pushability and torque transmission, enabling abetter operability to be achieved during insertion of the guide wire 1.

The material making up the proximal core member 3 is not subject to anyparticular limitation, provided it is an iron-containing alloy (eitheriron or steel). Preferred examples of materials that may be used includestainless steels (any SUS grade, such as SUS 304, SUS 303, SUS 316, SUS316L, SUS 316J1, SUS 316J1L, SUS 405, SUS 430, SUS 434, SUS 444, SUS429, SUS 430F and SUS 302) and piano wire.

When a stainless steel is used as the material making up the proximalcore member 3, the guide wire (A) of the invention is able to achieve aneven better pushability and torque transmission.

Piano wire is desirable because it has a high modulus of elasticity anda suitable elastic limit.

In the arrangement shown in the diagram, the proximal core member 3 hasa substantially constant outer diameter over substantially its entirelength. However, it may have places in the lengthwise direction wherethe outer diameter changes.

The coil 4 is a member composed of a fine, helically coiled wire, and isdisposed so as to cover the distal portion of the distal core member 2.In the illustrated arrangement, the distal portion of the distal coremember 2 has been inserted into the coil 4 to substantially the centerthereof. Moreover, the distal portion of the distal core member 2 hasbeen inserted therein without being in contact with the inside wall ofthe coil 4. The weld 14 is situated proximal to the proximal end of thecoil 4.

In the illustrated arrangement, the coil 4, which is shown in a statewhere no outside forces are applied thereto, has small gaps open betweenadjoining turns of the helically coiled wire. Alternatively, unlike theillustrated arrangement, when the coil is in a state where no outsideforces are applied thereto, it is also possible for the helically coiledwire to be tightly arranged without intervening gaps between adjoiningturns.

The coil 4 is preferably made of a metallic material. Examples ofsuitable metallic materials include stainless steels, superelasticalloys, cobalt alloys, precious metals such as gold, platinum andtungsten, and alloys containing such precious metals.

A radiopaque material such as a precious metal may be used for thispurpose. However, even if such a radiopaque material is not used,because the ferrous alloy of the invention is fluoroscopically visible,the guide wire 1 which includes the above-described distal core member 2will have fluoroscopic visibility and will thus be capable of insertioninto the body while tracking the position of the distal end underfluoroscopic control. Although the quality of the fluoroscopicvisibility is related to the density of the metallic material used, inthe case of Ti—Ni alloys, because the titanium which accounts forsubstantially half of the alloy has a relatively low density of 4.54g/cm³, slender portions such as the distal core member are sometimesdifficult to identify under x-ray fluoroscopy. By contrast, as shown inTable 1, the iron, nickel and cobalt which are the chief ingredients inthe ferrous alloys of the invention have respective densities of 7.86g/cm³, 8.85 g/cm³ and 8.8 g/cm³, which are much higher than the densityof titanium. Hence, such ferrous alloys have a higher density overallthan Ti—Ni alloys, making even the slender portions of the distal coremember easier to track under fluoroscopy.

The coil 4 may be made of a different material than the distal coremember 2 and the proximal core member 3 of the guide wire 1. The overalllength of the coil 4, while not subject to any particular limitation, ispreferably from about 5 mm to about 500 mm.

The proximal and distal ends of the coil 4 are respectively secured tothe distal core member 2 by fixing materials 11 and 12. In addition, anintermediate point (located closer to the distal end) of the coil 4 issecured to the distal core member 2 by a fixing material 13. Fixingmaterials 11, 12 and 13 are made of solder (braze). However, fixingmaterials 11, 12 and 13 are not limited to solder, and may even beadhesives. Methods for securing the coil 4 are not limited to methodsinvolving the use of a fixing material, and include also, for example,soldering. To prevent the guide wire from damaging the inside wall ofthe blood vessel, it is preferable for the distal endface of the fixingmaterial 12 to be rounded.

In the present embodiment, because such a coil 4 is provided thereon,the distal core member 2 is covered by the coil 4 and has a smallsurface area of contact. This enables the sliding resistance to bedecreased, which further enhances the operability of the guide wire 1.

In the present embodiment, a wire having a round cross-section is usedin the coil 4. However, the wire is not limited to this cross-sectionalshape; wires of other cross-sectional shapes, such as elliptical orquadrangular (particularly rectangular) shapes, may instead be used forthe same purpose.

In the guide wire 1, the distal core member 2 and the proximal coremember 3 are mutually coupled (fixed) by welding. In this way, a highcoupling strength (bond strength) can be achieved at the weld (junction)14 between the distal core member 2 and the proximal core member 3,enabling torsional torque and pushing forces from the proximal coremember 3 of the guide wire 1 to be reliably transmitted to the distalcore member 2.

It is preferable for the outer periphery of the weld 14 to be renderedsubstantially smooth by a method such as the subsequently describedprocedures (iii) and (iv).

In the present embodiment, the distal core member 2 has a connectingendface 21 for connection to the proximal core member 3, and theproximal core member 3 has a connecting endface 31 for connection to thedistal core member 2 that are each flat planes substantiallyperpendicular to the axial direction (lengthwise direction) of bothcores. This arrangement greatly facilitates the working operationsrequired to shape the connecting endfaces 21 and 31, enabling theabove-described effects to be achieved without further complicating theguide wire 1 manufacturing process.

Alternatively, unlike in the diagrams, the connecting endfaces 21 and 31may be angled with respect to a plane perpendicular to the axialdirection (lengthwise direction) of both core members 2 and 3, or mayeven be concave or convex.

Examples of methods for welding together the distal core member 2 andthe proximal core member 3, while not subject to any particularlimitation, include laser welding, and butt resistance weldingtechniques such as upset welding and flash welding. A butt resistancewelding technique is preferred for achieving the weld 14 having a higherbond strength.

Next, referring to FIG. 2, the steps involving in joining together thedistal core member 2 and the proximal core member 3 by upset welding,which is one type of butt resistance welding technique, are described.FIG. 2 shows steps (i) to (iv) involved in joining together the distalcore member 2 and the proximal core member 3 by upset welding.

Step (i) shows a distal core member 2 and a proximal core member 3 whichare secured to (mounted on) a butt welding machine (not shown).

In step (ii), the connecting endface 21 on the proximal end of thedistal core member 2 and the connecting endface 31 on the distal end ofthe proximal core member 3 are brought into pressurized contact by thebutt welding machine while a predetermined voltage is applied. Underthis pressurized contact, a molten layer forms at the areas in contact,thereby securely connecting the distal core member 2 with the proximalcore member 3.

In step (iii), a protruding area that has arisen at the place ofconnection (weld 14) due to deformation by the pressurized contact isremoved, thereby making the outer periphery of the weld 14 substantiallysmooth. Methods for removing such a protruding area include grinding,polishing, and chemical treatment such as etching.

Next, in step (iv), areas of the distal core member 2 which are distalto the place of connection (weld 14) are ground or polished so as toform a tapering diameter portion 15 in which the outer diametergradually decreases toward the distal end.

In cases where the proximal end of the tapering diameter portion 15 isproximal to the weld 14, step (iv) may be carried out without firstcarrying out step (iii).

In the guide wire (A) of the embodiment, the proximal core member ismade of a SUS grade or other type of stainless steel. After the distalcore member and the proximal core member have been welded together, itis advantageous to subject the proximal end of the distal core member tohardening treatment.

Because the distal core member is more flexible than the proximal coremember, when the outer diameters of both core members are about thesame, the difference between the rigidities of the two core members oneither side of the weld is large. However, it appears that thedifference in rigidities can be reduced by subjecting the proximal endof the distal core member to hardening treatment.

The fact that the distal core member made of the ferrous alloy of theinvention has a broader superelasticity region than Ti—Ni alloys, takentogether with the difference in rigidities, means that bending occurs atthe distal side of the weld, leading to stress concentration at theweld. However, it is believed that by carrying out hardening treatmentas described above and thus reducing the difference between therigidities of the two core members, stress concentration at the weldwill be suppressed so that even when strongly flexed, the guide wirewill curve smoothly without localized bending.

Examples of hardening treatment include aging and peening. Whenhardening is carried out by aging, hardening occurs by the precipitationof intermetallic compounds such as Ni₃Al on the proximal side of thedistal core member. The amount of precipitate on the proximal side ofthe distal core member is higher than in untreated areas. In peening,the surface layer can be work-hardened, enhancing the rigidity. In bothof these hardening treatments, it is preferable to carry out treatmentso that the flexibility gradually increases toward the distal end. Also,it is desirable to reduce the diameter at the distal end of the proximalcore member so as to make it more flexible than other portions of theproximal core member and thereby reduce the difference in rigidity.

In the guide wire 1 shown in FIG. 1, the core 10 has a coating 5 whichcovers all or part of the outer peripheral surface (outside surface) ofthe guide wire 1. This coating 5 may be formed for a number of purposes,one of which is to reduce friction (sliding resistance) by the guidewire 1, thus improving slideability and in turn enhancing theoperability of the guide wire 1.

To achieve this purpose, it is preferable for the coating 5 to be madeof a material capable of lowering friction. In this way, frictionalresistance (sliding resistance) with the inside wall of the catheterused together with the guide wire 1 decreases, improving slideabilityand resulting in even better operability of the guide wire 1 within thecatheter. Also, because the guide wire 1 has a lower sliding resistance,when the guide wire 1 is moved and/or rotated inside the catheter,kinking and twisting of the guide wire 1, especially kinking andtwisting in the vicinity of the weld, can be more reliably prevented.

Examples of such materials capable of reducing friction includepolyolefins such as polyethylene and polypropylene, polyvinyl chloride,polyesters (e.g., PET, PBT), polyamides, polyimides, polyurethanes,polystyrenes, polycarbonates, silicone resins, fluorocarbon resins(e.g., PTFE, ETFE), and various other elastomers, as well as compositematerials thereof.

Of the above, when a fluorocarbon resin (or a composite materialcontaining the same) is used, the frictional resistance (slidingresistance) between the guide wire 1 and the catheter inside wall can bemore effectively reduced, enabling the slideability to be improved, andthus making it possible to achieve an even better guide wire 1operability within the catheter. Moreover, it is possible in this way tomore reliably prevent kinking and twisting of the guide wire 1, andespecially kinking and twisting in the vicinity of the weld, when theguide wire 1 is moved and/or rotated inside the catheter.

When a fluorocarbon resin (or a composite material containing the same)is used, the resin material is generally coated onto the core 10 in aheated state by a process such as baking or blowing, thereby resultingin an exceptionally good adhesion between the core 10 and the coating 5.

The fluorocarbon resin coated in this way onto the core 10 is generallybaked at about 300 to 400° C., although the ferrous alloy of theinvention is not readily affected by such temperatures and is thusresistant to changes in its properties.

When the coating 5 is composed of a fluorocarbon resin such as PTFE orPFA, another coating (undercoat) may also be provided between the core10 and the coating 5. Adhesion between the coating and the undercoat canbe improved by having the undercoat be composed of a mixture of aheat-resistance resin such as a polyimide, polyamide or polyamideimidewith a fluorocarbon resin such as PTFE or PFA. In this way, the core 10made of a ferrous alloy having a wide superelasticity region can beprovided with an ability to navigate bends and twists in the vasculatureand, owing to the coating 5 made of a fluorocarbon resin, can also beendowed with a good slideability.

Other preferred materials capable of reducing friction includehydrophilic materials.

Illustrative examples of such hydrophilic materials includecellulose-based polymeric substances, polyethylene oxide-based polymericsubstances, maleic anhydride-based polymeric substances (e.g., maleicanhydride copolymers such as methyl vinyl ether-maleic anhydridecopolymers), acrylamide-based polymeric substances (e.g.,polyacrylamides, polyglycidyl methacrylate-dimethylacrylamide(PGMA-DMAA) block copolymers), water-soluble nylons, polyvinyl alcoholsand polyvinylpyrrolidone.

Such hydrophilic materials often exhibit lubricity on wetting (waterabsorption), lowering the frictional resistance (sliding resistance)with the inside walls of the catheter used together with the guide wire1. As a result, the slideability of the guide wire 1 is improved, andthe guide wire 1 has an even better operability within the catheter.

The coating 5 is preferably made of a thermoplastic elastomer. Becausethe distal core member is made of the ferrous alloy of the invention, itcan be deformed in a broader superelasticity region than Ti—Ni alloys.However, even when the distal core member made of the inventive ferrousalloy is subjected to large deformation, because the coating is made ofa thermoplastic elastomer, stretching by the coating conforms todeformation of the distal core member, as a result of which the coatingdoes not readily peel from the distal core member.

Examples of the thermoplastic elastomer include polyurethane elastomersand polyamide elastomers. It is desirable to coat the outside surface ofthe coating 5 made of a thermoplastic elastomer with a hydrophilicmaterial.

While such a coating 5 may be formed either over the entire length ofthe core 10 or over a portion of the core 10 in the lengthwisedirection, it is preferable to form the coating in places that includethe weld so as to cover the weld 14. In this way, even in the unlikelyevent that bumps, flash or the like should arise on the outer peripheryof the weld 14, such areas are covered by the coating 5, enablingslideability to be ensured. Moreover, the coating 5 has a substantiallyuniform outer diameter, further enhancing the slideability.

The coating 5 has an average thickness which, while not subject to anyparticular limitation, is preferably from about 1 μm to about 20 μm, andmore preferably from about 2 μm to about 10 μm. If the coating 5 is toothin, the purpose of forming the coating 5 may not be fully achieved,and peeling of the coating 5 may arise. On the other hand, the coating 5which is too thick may interfere with the properties of the wire and mayalso lead to peeling of the coating 5.

In the guide wire (A) of the embodiment, treatment (e.g., chemicaltreatment, heat treatment) may be additionally carried out to improveadhesion of the coating 5 to the outside peripheral surface of the core10, or to provide an intermediate layer capable of enhancing adhesion ofthe coating 5.

Next, a second embodiment of the guide wire (A) is described whilereferring to FIG. 3, with particular reference to those features whichdiffer from the foregoing first embodiment of the invention.Descriptions of like features are omitted.

In the guide wire 1 shown in FIG. 3, a first coating 5 has a distal endlocated at a position proximal to the proximal end of the coil 4, and asecond coating 6 differing from the first coating 5 is formed distal tothe first coating 5.

The guide wire (A) may thus have two or more coatings. Moreover, thecoatings may partially or completely overlap.

The second coating 6 is provided so as to cover part or all of the coil4. In the arrangement shown in FIG. 3, the second coating 6 covers theentire coil 4.

The material making up such a second coating 6 may be the same as ordifferent from those mentioned above in connection with the earlierdescribed coating 5. Examples of suitable materials for this purposeinclude plastics, and more specifically, polyolefins such aspolyethylene and polypropylene, polyvinyl chloride, polyesters (e.g.,PET, PBT), polyamides, polyimides, polyurethanes, polystyrenes,polycarbonates, fluorocarbon resins, silicone resins, silicone rubbers,and various other elastomers (e.g., polyamide-based and polyester-basedthermoplastic elastomers).

As noted above, the materials making up the first coating 5 and thesecond coating 6 are not subject to any particular limitation. However,it is preferable for the first coating to be made of a silicone resin(or a composite material including the same), and for the second coating6 to be made of a fluorocarbon resin (or a composite material includingthe same).

In this way, the coatings can be made to possess both the advantages ofsilicone resins and the advantages of fluorocarbon resins describedabove. That is, by combining the first coating 5 material and the secondcoating 6 material in the above way, the entire guide wire 1 can beendowed with a sufficient slideability and an excellent operabilitywhile retaining a good bond strength between the distal core member 2and the proximal core member 3 at the weld 14.

Alternatively, in cases where the first coating 5 is made of afluorocarbon resin (or a composite material containing the same) and thesecond coating 6 is made of a polyurethane resin (or a compositematerial containing the same), substances having good angiographicproperties may be mixed into the polyurethane resin, enabling thevisibility under fluoroscopic imaging to be improved. Also, by havingthe polyurethane coating cover the underlying metal, safety is enhancedbecause, for example, the risk of intravascular failure by the wire canbe avoided. The hydrophilic material coated onto the surface of thesecond coating 6 improves lubricity. Fluorocarbon resin-covered areasare desirable because they have a good slideability within the catheter.

In cases where the first coating 5 is made of a hydrophobic resin andthe second coating 6 is made of a hydrophilic resin, the guide wire 1has an especially good slideability within the catheter and an excellentthreadability through vascular lumens.

The second coating has an average thickness which, while not subject toany particular limitation, is preferably from about 1 μm to about 20 μm,and more preferably from about 2 μm to about 10 μm. The second coating 6has a thickness which may be the same as or different from that of thefirst coating 5.

The guide wire (A) of the present embodiment may be one that is notfitted with a coil 4, in which case the above-described second coating 6may or may not be provided in the same places as indicated above.

That is, the guide wire (A) may be one which lacks a coil 4 and in whicha coating composed of a plastic such as one of those mentioned above isprovided in place of the coil 4. For example, the guide wire (A) mayhave a plastic coating which is made of the same material as theabove-described first coating 5 or second coating 6, and which isprovided as a tubular member for covering the distal portion of theabove-described core. It is preferable for the plastic coating to becomposed of a polyurethane resin (or a composite material containing thesame). The reason is that substances having good angiographic propertiesmay be mixed into the polyurethane resin, enabling the visibility underfluoroscopic guidance to be improved. Moreover, it is also preferablefor the plastic coating to be composed of a polyurethane resin and tohave a coating 5 made of a fluorocarbon resin. Here too, the reason isthat substances having good angiographic properties may be mixed intothe polyurethane resin, enabling the visibility under fluoroscopicguidance to be improved. An additional reason is that by havingpolyurethane cover the metal, doctors can satisfy because, for example,the occurrence of intravascular failure by the wire can be avoided.Also, by coating a hydrophilic material onto the surface of thepolyurethane resin, a good lubricity is achieved. The first coating 5made of a fluorocarbon resin has a good slideability within thecatheter.

In the arrangement shown in FIG. 3, the distal end of the first coating5 and the proximal end of the second coating 6 are bonded together andformed so that the two layers are continuous. Alternatively, the distalend of the first coating 5 and the proximal end of the second coating 6may be located away from each other, or the first coating 5 and thesecond coating 6 may partially overlap.

Next, a preferred embodiment of another guide wire (B) according toanother aspect of the present invention is described in detail whilereferring to FIG. 4.

FIG. 4 is a longitudinal sectional view of an embodiment of a guide wire(B) according to another aspect of the invention. For the sake ofconvenience, in FIG. 4 and in subsequently described FIG. 5, the portionof the guide wire appearing the right side of the diagram is referred toherein as the “proximal side” and the portion of the guide wireappearing on the left side of the diagram is referred to as the “distalside.” In FIG. 4 and subsequently described FIG. 5, the guide wire isshown schematically so as to appear shortened in the length directionand exaggerated in the thickness direction, as a result of which theratio between the length direction and the thickness direction as shownin the diagrams differs considerably from the actual ratio.

The guide wire 101 shown in FIG. 4 is a catheter guide wire for use ininsertion into a catheter. The guide wire 1 has a core 102, a proximaltube 103 which is disposed on the proximal side of the core 102 andcovers the proximal side of the core 102, and a helical coil 104. Theproximal side of the core 102 is thinner than the distal side, with theproximal tube 103 being disposed so as to cover this thin portion (alsoreferred to below as the “small diameter portion 110”).

The guide wire 101 has a total length which, while not subject to anyparticular limitation, is preferably in a range of from about 200 mm toabout 5,000 mm. The core 102 has an outer diameter (outer diameter ofportion having a constant outer diameter) which, while not subject toany particular limitation, is preferably from about 0.1 mm to about 1.0mm at the small diameter portion 110 on the proximal side, and ispreferably from about 0.2 mm to about 1.2 mm in other portions on thedistal side. The proximal tube 103 has an outer diameter which, whilenot subject to any particular limitation, is preferably from about 0.2mm to about 1.2 mm. It is preferable for the inner diameter of theproximal tube 103 and the outer diameter of the small diameter portion110 of the core 102 to be substantially the same and for the proximaltube 103 to be in close contact with the core 102.

At least the distal end of the core in the guide wire (B) is made of theferrous alloy of the invention. In the present embodiment, all of thecore 102 is a wire with elasticity that is made of the ferrous alloy ofthe invention. The core 102 has a length which, while not subject to anyparticular limitation, is preferably in a range of from about 200 mm toabout 5,000 mm.

Also, in the present embodiment, apart from the small diameter portion110 thereof, the core 102 has an outer diameter which is constant for agiven length from the proximal end thereof, then tapers toward thedistal end starting at some intermediate point. This latter portion isreferred to herein as the tapering diameter portion 115. By having sucha tapering diameter portion 115, the rigidity (flexural rigidity,torsional rigidity) of the core 102 can be gradually reduced in thedistal direction. As a result, the guide wire 101 has a good flexibilityin the distal portion, thus improving the safety and the ability of theguide wire to navigate a blood vessel, and preventing undesirableeffects such as kinking.

In the illustrated arrangement, the tapering diameter portion 115 isformed as part of the core 102 outside of the small diameter portion110, although the core 102 may be in its entirety a tapering diameterportion 115. The angle of taper (rate of reduction in the outerdiameter) on the tapering diameter portion 115 may be constant along thelength of the wire or there may be points along the length where theangle of taper changes. For example, a plurality of places where theangle of taper is relatively large and a plurality of places where theangle of taper is relatively small may be repeatedly formed inalternation.

Or the core 102 may have a portion with an outer diameter that isconstant in the lengthwise direction, either partway along the taperingdiameter portion 115 or distal to the tapering diameter portion 115. Forexample, the core 102 may have formed, at a plurality of places in thelengthwise direction, tapered portions on which the outer diametertapers in the distal direction, and may have formed, between one suchtapered portion and another such tapered portion, a portion having aconstant outer diameter in the lengthwise direction. The effectsachieved in such cases are the same as those indicated above.

Alternatively, unlike the arrangement shown in FIG. 4, it is possiblefor the guide wire 101 to have a configuration in which the proximal endof the tapering diameter portion 115 is situated partway along theproximal tube 103; i.e., the tapering diameter portion 115 is formed soas to straddle the interface between the core 102 and the proximal tube103.

The material making up the core 102 is the ferrous alloy of theinvention, which is described more fully later in the specification.

In the guide wire (B), when a portion of the core other than the distalportion is made of a material other than the ferrous alloy of theinvention, the other material is not subject to any particularlimitation. For example, the material may the same as that used in thesubsequently described proximal tube 103.

The proximal side of the core 102 is covered by the proximal tube 103.The proximal tube 103 is composed of metal wire having a higherelasticity than the core 102. The proximal tube 103 has a length which,while not subject to any particular limitation, is preferably in a rangeof from about 20 mm to about 4,800 mm.

In FIG. 4, the core 102 has on the proximal side thereof the smalldiameter portion 110 which is covered by the proximal tube 103. However,in the guide wire (B), the core 102 need not necessarily have the smalldiameter portion 110. That is, the core 102 may have a portion otherthan the tapering diameter portion 115 which has an outer diameter thatis constant, which outer diameter is substantially the same as the innerdiameter of the proximal tube 103.

The proximal tube 103 is made of a material having larger moduli ofelasticity (Young's modulus (longitudinal elastic modulus), modulus ofrigidity (modulus of transverse elasticity), volumetric elastic modulus)than the material making up the core 102. In this way, a suitablerigidity (flexural rigidity, torsional rigidity) can be achieved in theproximal tube 103, giving the guide wire 101 a good stiffness thatenhances the pushability and torque transmission, enabling a betteroperability to be achieved during insertion.

The material making up the proximal tube 103 is not subject to anyparticular limitation, and may of the same type as the material makingup the proximal core member 3 in the guide wire (A). Preferred examplesof the material are the same as those mentioned above for the materialused in the proximal core member 3 in guide wire (A). Alternatively, thematerial may be a cobalt alloy.

Preferred combinations of the core 102 and the proximal tube 103 areones in which the core 102 is made of a ferrous alloy of the inventionand the proximal tube 103 is made of a stainless steel or a cobaltalloy. The above-indicated effects can be more distinctly achieved inthis way. Alternatively, bringing a pipe-shaped proximal tube made of aniron-containing material such as stainless steel into close contact withthe proximal side of the core made of the ferrous alloy of the inventionis desirable because the ability for the core and the proximal tube tomutually bond tends to increase as a result. The diffusion therebetweenof iron present in the core and the proximal tube is thought to be animportant factor in this effect.

The cobalt alloy referred to here has a high modulus of elasticity and asuitable elastic limit. The cobalt alloy used may be any alloy whichcontains cobalt as a constituent element, although the use of an alloycontaining cobalt as a primary component (cobalt-based alloys referherein to alloys in which, of the elements making up the alloy, thecontent of cobalt in terms of weight ratio is the highest) is preferred,and the use of a Co—Ni—Cr alloy is more preferred. Alloys having such acomposition, when used as the material making up the proximal tube 103,render the above-described effects of the embodiment even more striking.Moreover, because alloys of such a composition have a plasticity evenwhen deformed at room temperature, they can be easily deformed into adesired shape at the time of use, for example. Alloys with such acomposition have a high coefficient of elasticity and are cold-formableeven at a high elastic limit. Because of the high elastic limit, asmaller diameter can be achieved while fully preventing buckling fromoccurring. In addition, the alloy can be provided with a flexibility anda rigidity sufficient for insertion to the target site.

Preferred Co—Ni—Cr alloys include alloys having a composition whichincludes from 28 to 50 wt % cobalt, from 10 to 30 wt % nickel, and from10 to 30 wt % chromium, with the balance being iron; and alloys in whicha portion of the above has been substituted with other elements(substitution elements). The inclusion of substitution elements elicitseffects that are specific to the particular elements. For example, byincluding one or more elements selected from among titanium, niobium,tantalum, beryllium and molybdenum as substitution elements, thestrength of the proximal tube 103 can be improved even further. When anelement other than cobalt, nickel and chromium is included, the contentthereof (i.e., of the substitution elements overall) is preferably notmore than 30 wt %.

It is also possible for portions of the cobalt, nickel and chromium tobe substituted with other elements. For example, a portion of the nickelmay be substituted with manganese to achieve, for example, a furtherimprovement in workability. Also, a portion of the chromium may besubstituted with molybdenum and/or tungsten so as to, for example,further improve the elastic limit. Of such Co—Ni—Cr alloys, the use ofmolybdenum-containing alloys, i.e., Co—Ni—Cr—Mo alloys, is especiallypreferred.

The Co—Ni—Cr alloys are exemplified by alloys having the followingcompositions (the numbers indicate percent by weight):

-   -   40Co-22Ni-25Cr-2Mn-0.17C-0.03Be-balance Fe    -   40Co-15Ni-20Cr-2Mn-7Mo-0.15C-0.03Be-balance Fe    -   42Co-13Ni-20Cr-1.6Mn-2Mo-2.8W-0.2C-0.04Be-balance Fe    -   45Co-21Ni-18Cr-1Mn-4Mo-1Ti-0.02C-0.3Be-balance Fe    -   34Co-21Ni-14Cr-0.5Mn-6Mo-2.5Nb-0.5Ta-balance Fe

As used herein, “Co—Ni—Cr alloy” encompasses the above alloys.

In the illustrated arrangement, the proximal tube 103 has asubstantially constant outer diameter over substantially its entirelength, although it may have sites thereon where the outer diameterchanges in the lengthwise direction.

The positioning, material and other features of the coil 104 may be thesame as for the coil 4 in the above-described guide wire (A). Preferredfeatures of the coil 104 are likewise the same as described for the coil4 in guide wire (A).

In the guide wire 101 of the present embodiment, the core 102 and theproximal tube 103 are coupled (fixed) to each other by placing themtogether, then subjecting them to a drawing operation or the like. Inthis way, a high coupling strength (bond strength) can be achieved atthe connection between the core 102 and the proximal tube 103, making itpossible in the guide wire 101 for torsional torque and pushing forcefrom the proximal tube 103 to be reliably transmitted to the core 102.

The core 102 (small diameter portion 110) and the proximal tube 103 maybe joined together by spot-like laser welding to the outside surface ofthe proximal tube 103 or by welding in the lengthwise direction as in aseam pipe. This is preferable because the bondability between the core102 and the proximal tube 103 can be enhanced. In cases where a proximaltube composed of stainless steel is used is in particular, this is evenmore preferable because the bondability is further enhanced. The reasonis that, in the guide wire (B), the core 102 is made of the ferrousalloy of the invention. When the core is made of, unlike the guide wire(B), a Ti—Ni alloy for example, brittle intermetallic compounds composedof titanium and iron form at the joint between the proximal tube made ofstainless steel and the core. In such a case, the bondability betweenthe core and the proximal tube is low. In the guide wire (B), by using aproximal tube composed of stainless steel, such brittle intermetalliccompounds do not readily form in the welded area, resulting in a strongbond.

The guide wire (B) has a coating 105 which covers part or all of theoutside peripheral surface of the core 102 and the proximal tube 103.This coating 105 may be formed for various purposes, one example ofwhich is to reduce friction (sliding friction) by the guide wire 101,improve slideability, and thereby improve the operability of the guidewire 101.

To this end, it is preferable for the coating 105 to be made of amaterial capable of reducing friction. Frictional resistance (slidingresistance) with the inside wall of the catheter used together with theguide wire 101 is thereby reduced, enhancing the slideability and thusresulting in a better guide wire 101 operability within the catheter.Moreover, because the sliding resistance of the guide wire 101 is lower,when the guide wire 101 is moved and/or rotated within the catheter,kinking and twisting of the guide wire 101, particularly kinking andtwisting in the vicinity of the weld, can be reliably prevented.

Materials capable of being used as the coating 5 in the above-describedguide wire (A) may be used as such friction-lowering materials in thepresent embodiment. The preferred examples of such materials mentionedabove in connection with the guide wire (A) are preferable for use inthe present embodiment as well.

The places where the coating 105 are formed and the thickness of thecoating 105 are likewise the same as described above with regard to thecoating 5 used in the earlier described guide wire (A).

In the guide wire (B), treatment (e.g., chemical treatment, heattreatment) for the purpose of improving adhesion of the coating 105 maybe applied to the outer peripheral surface of the core 102 and/or theproximal tube 103. Alternatively, an intermediate layer capable ofimproving the adhesion of the coating 105 may be provided thereto.

Next, an embodiment of a guide wire (C) is described while referring toFIG. 5, with particular reference to those features which differ fromthe foregoing embodiments of the guide wires (A) and (B). Descriptionsof like features are omitted.

In a guide wire 201 shown in FIG. 5, a distal core member 202 and aproximal core member 203 are composed of ferrous alloys. At least partof the surface of the proximal core member 203 has a first coating 205therein made of a fluorocarbon resin.

A second coating 206 differing from the first coating 205 is formeddistal to the first coating 205. The first coating 205 is provided so asto cover all or part of the proximal core member 203. In the illustratedarrangement, the first coating 205 covers substantially all of theproximal core member 203.

The second coating 206 is provided so as to cover all or part of thedistal core member 202. In the illustrated arrangement, the secondcoating 206 covers substantially all of the distal core member 202.

The first coating 205 is made of a fluorocarbon resin such as PTFE orPFA. In addition, a separate coating (undercoat) may be provided betweenthe proximal core member 203 and the first coating 205. By having theundercoat be composed of a resin such as a polyimide, polyamide orpolyamideimide which is capable of resisting even the meltingtemperature of the fluorocarbon resin, the resistance to separation(peel resistance) of the first coating 205 made of a fluorocarbon resinfrom the proximal core member 203 can be improved. Alternatively, thepeel resistance between the first coating 205 and the undercoat can beimproved by mixing a fluorocarbon resin such as PTFE and PFA into theundercoat. In this way, the proximal core member 203 made of a ferrousalloy having an extremely broad superelasticity region is capable offollowing twists and turns, along with which a good slideability can beachieved on account of the first coating 205 made of a fluorocarbonresin.

It is also possible to roughen the surface of the proximal core member203 so as to improve the peel resistance of the first coating 205.

The second coating 206 is preferably made of a thermoplastic elastomer.Because the distal core member 202 is made of the subsequently describedferrous alloy, it can be deformed over a broader superelasticity regionthan Ti—Ni alloys. However, even when the distal core member made of thesubsequently described ferrous alloy is subjected to a largedeformation, because the second coating 206 is made of a thermoplasticelastomer, stretching by the coating conforms to deformation of thedistal core member, as a result of which the coating does not readilypeel from the distal core member.

Examples of the thermoplastic elastomer include polyurethane elastomersand polyamide elastomers. It is desirable for a hydrophilic material tobe coated onto the outside surface of the second coating 206 made of athermoplastic elastomer. In addition, to improve the peel resistance, itis possible to provide between the second coating 206 and the distalcore member 202 a resin layer having a higher resistance to peeling fromthe distal core member 202 than the second coating 206. Also, theresistance of the coating 206 to peeling from the distal core member 202can be improved by roughening the surface of the distal core member 202.

The first coating 205 has an average thickness which, while not subjectto any particular limitation, is preferably from about 1 μm to about 20μm, and more preferably from about 2 μm to about 10 μm.

The distal core member 202 has a tapering diameter portion 215, and theproximal core member 203 has a tapering diameter portion 216. The secondcoating 206 is coated to a uniform outer diameter along the length ofthe tapering diameter portion 215. The first coating 205 is coated to asubstantially uniform diameter at the tapering diameter portion 216 andproximal thereto.

In an alternative arrangement, the second coating 206 on the guide wire(C) may cover the proximal core member 203. In such a case, the firstcoating 205 need not be provided. Even in such an arrangement, tofurther improve the peel resistance, it is possible to provide, betweenthe second coating 206 and the proximal core member 203, a resin layerhaving a higher resistance to peeling from the proximal core member 203than the second coating 206. Also, the peel resistance of the secondcoating 206 from the proximal core member 203 can be improved byroughening the surface of the proximal core member 203.

In the arrangement shown in FIG. 5, the distal end of the first coating205 and the proximal end of the second coating 206 are located away fromeach other. However, the two layers may be formed so as to becontinuous, or the first coating 205 and the second coating 206 maypartially overlap.

Next, the stent according to further aspect of the present invention isdescribed.

The stent has a body composed of the ferrous metal of the invention. Forexample, the stent body may have on the surface thereof (inside surfaceand/or outside surface) a film composed of a biocompatible material.Alternatively, a body alone that does not have such a film thereon isalso acceptable and within the scope of the invention.

Next, the shape of the stent is described using preferred embodimentsshown in the attached diagrams.

FIG. 6 is a front view showing the shape of a stent according to apreferred embodiment of the invention. FIG. 7 is a development view ofthe stent shown in FIG. 6. FIG. 8 is a development view of the stentshown in FIG. 6, in the contracted state. FIG. 9 is a partial, enlargedview of the stent shown in FIG. 6.

The stent 301 of the present embodiment is a stent having a plurality ofundulating rings 302. Each undulating ring 302 has a plurality offirst-side bends with an apex 302 a on a first side in the axialdirection of the stent 301, and a plurality of second-side bends with anapex 302 b on a second side in the axial direction of the stent 301.Undulating rings 302 which are mutually adjacent in the axial directionof the stent 301 have a shared linear segment 321 which has a startingend 322 at or near one apex 302 b of a second-side bend in theundulating ring 302 on the first side in the axial direction of thestent 301 and which has a terminal end 323 between the apex 302 b in thesecond-side bend and an apex 302 a in a first-side bend, which sharedlinear segment 321 unites the mutually adjoining undulating rings.

The stent has an arrangement which includes no elements provided solelyas links and is composed solely of elements that effectively contributea force of expansion, because it is made up of a plurality of rings inthe form of mutually adjoining undulating rings that are united by thepresence of partly shared segments.

The stent in the present embodiment is a so-called self-expanding stentwhich is formed in a substantially cylindrical shape, which is in acontracted state at the time of insertion into the body, and which, whenit has been placed within the body, is capable of returning to itsoriginal expanded shape prior to contraction. FIG. 6 shows theappearance of the stent 301 when expanded.

In the stent shown in FIG. 6, the number of undulating rings 302 whichform the stent 301 is eleven. The number of undulating rings 302 differsdepending on the length of the stent, but is preferably from 2 to 150,and more preferably from 5 to 100.

Each undulating ring 302 has a plurality of first-side bends having anapex on a first side in the axial direction of the stent 301 and aplurality of second-side bends having an apex on a second side in theaxial direction of the stent 301, and is composed of an endlessundulating element which is annularly continuous. The first-side bendsand the second-side bends in the ring 302 are alternately formed, andthus are each present in the same number. In the stent shown in FIG. 6,the number of first-side bends (second-side bends) in a singleundulating ring 302 is nine. The number of first-side bends (second-sidebends) is preferably from 4 to 20, and more preferably from 6 to 12.Moreover, the linear elements which form the annular ring 302 in thestent of the present embodiment are almost always curved; rectilinearelements are very rare. Because the linear elements forming the ring 302thus have a sufficient length, a large expansion force acts duringexpansion. The undulating ring 302 has a length in the axial directionof preferably from 1 to 10 mm, and more preferably from 1.5 to 5 mm.

In the stent 301 of the present embodiment, as shown in FIGS. 6 to 9,each of the undulating rings 302 has a large wave that forms a salientfirst-side apex 302 a 1 which projects out further on the first sidethan the apices 302 a on other first-side bends, and that forms asalient second-side apex (which, in the present embodiment, coincideswith a starting end) 322 which projects out further on the second sidethan the apices of the other second-side bends. Moreover, in the presentembodiment, the undulating rings each have a plurality of large waves.In the present stent, one ring has nine first-side bends, and threelarge waves are provided within a single ring. The three large waves areformed so as to be spaced apart at substantially equal angles withrespect to the center axis of the stent 301.

Undulating rings 302 which are mutually adjacent in the axial directionof the stent 301 have a shared linear segment 321 with a starting end322 at or near one apex 302 b of a second-side bend in the undulatingring 302 on the first side of the stent 301 in the axial directionthereof and having a terminal end 323 between the apex 302 b in thesecond-side bend and an apex 302 a in a first-side bend on the same ring302, which shared linear segment 321 unites the mutually adjacentundulating rings.

Specifically, the shared linear segment 321 has, as the starting end 322thereof, one apex 302 b on a second-side bend in an annular ring 302 ona first side of the stent 301 in the axial direction thereof. Thestarting end 322 and the apex 302 b are the same. The shared linearsegment 321 has a terminal end 323 located between the foregoing apex302 b (which is also the starting end 322) and the apex 302 a of afirst-side bend that is continuous thereto. In particular, in thepresent embodiment, the shared linear segment 321 has a terminal end 323located substantially near the midpoint between the foregoing apex 302 b(which is also the starting end 322) and the apex 302 a of thefirst-side bend that is continuous thereto. The terminal end 323 ispreferably located at the midpoint, although it may be located at anyapex-side position from about 1/100 to about 49/100 of the total lengthbetween the foregoing apex 302 b (which is also the starting end 322)and the apex 302 a of the first-side bend that is continuous thereto. Inthe present embodiment, the terminal end 323 is preferably positionedsomewhat to the apex 302 a side from the midpoint.

The stent 301, owing to the above-described arrangement, has a startingend branch which forms the starting end site for the shared linearsegment 321 and has a terminal end branch which forms the terminal endsite for the shared linear segment 321. Specifically, the starting endbranch has a shape which divides into two legs facing the first side atthe starting end 322 as the branch point, and the terminal end branchhas a shape which divides into two legs facing the second side at theterminal end 323 as the branch point.

Moreover, in the stent 301 of the present embodiment, the intervalbetween the salient first-side apex 302 a 1 and the salient second-sideapex (which is the same as the starting end 322) in a large wave is alinear segment which is longer than connecting linear segments betweenother apices. As noted above, the second-side end of this long linearsegment is the starting end of the shared linear segment. It should alsobe noted that, in the present embodiment, the shared linear segment 321is formed on part of a large wave.

In the stent 301 of the present embodiment, as shown in FIG. 7, eachundulating ring 302 has a short linear segment 326 which connectsbetween the terminal end 323 of the shared linear segment 321 and theapex 302 a of a first-side bend. Moreover, as shown in FIG. 7, the ring302 which is united by the shared linear segment 321 with the ring 302having the above short linear segment 326 has a short linear segment 325which connects between the starting end 322 of the shared linear segment321 and the apex 302 b of a second-side bend and has a long linearsegment 324 which connects between the terminal end 323 of the sharedlinear segment 321 and another apex 302 b of a second-side bend.Therefore, the interval between the salient first-side apex (which isthe same as the terminal end 323) on a large wave and a salientsecond-side apex (which coincides with the starting end 322 of thelinear segment shared with the adjoining ring on the second side) formsa long linear segment. That is, in the present stent 301, shared linearsegments 321 which are mutually adjoining in the axial direction have aconfiguration wherein, as viewed from the first side of the stent 301 inthe axial direction, the terminal end 323 of one shared linear segment321 and the starting end 322 of the other shared linear segment 321adjacent thereto are connected by a long linear segment 324.Accordingly, as shown in FIG. 7, in the present stent 301, a zigzagarrangement composed of repeated long linear segments 324 and sharedlinear segments 321 forms a helix that extends from one end to the otherend of the stent 301.

Moreover, because this stent has no so-called links, there are noobstacles to curvature or decreases in expansion force caused by suchlinks. As a result, the entire stent exhibits a uniform expansionretaining force.

Also, the stent 301 of the present embodiment has a plurality of sharedlinear segments 321 between mutually adjacent undulating rings.Specifically, three shared linear segments 321 are provided betweenmutually adjacent undulating rings. Moreover, the three shared linearsegments 321 are formed at intervals that are spaced at substantiallyequal angles with respect to the center axis of the stent 301.

Furthermore, in the stent 301 of the present embodiment, short linearsegments 325 which connect between the starting ends 322 of the sharedlinear segments 321 and the apices 302 b of second-side bends, are notcontinuous in the axial direction of the stent 301, and a plurality ofthe short linear segments 325 are formed so as to be substantiallyrectilinear. Also, in the stent 301 of the present embodiment, linearsegments other than the above-described short linear segments 325 and326 (i.e., long linear segments and other linear segments) have near thecenter thereof, as shown in FIG. 9, areas of inflection 332 where thelinear segment advances in a direction that changes somewhat whileremaining substantially parallel. Such areas of inflection 332 increasethe length of the linear segments and also increase the expansion force.

Also, in the stent 301 of the present embodiment, the long linearsegments 324 have a length (i.e., the length between the terminal end323 of a shared linear segment 321 and the starting end 322 of anothershared linear segment 321) which is somewhat longer than the combinedlength of a shared linear segment 321 and a short linear segment 325(i.e., the length from the terminal end 323 of the shared linear segment321 to the apex 302 b beyond the starting end 322 thereof). This makesit possible to prevent the apex 302 b from coming too close to thelinear segment 333 of an adjoining ring (specifically, an ordinarylinear segment which connects one apex 302 a with another apex 302 b,and which has no shared linear segment and no branch point), enablingdeviations in width within the closed spaces formed by linear segments(in the present embodiment, the closed spaces formed by connecting the“V” and “M” shapes, as shown in FIG. 7) to be minimized, so that a highexpansion holding force is achieved.

Also, as shown in FIG. 7, the apices 302 a of first-side bends in theundulating rings 302 intrude on the spaces that form between the apices302 b of second-side bends on one of the neighboring undulating rings,and the apices 302 b of the second-side bends in the undulating rings302 intrude on the spaces that form between the apices 302 a offirst-side bends on the other neighboring undulating ring. Thisarrangement enables the length of the linear segments making up thestent to be increased, and also makes it possible to reduce the surfaceareas of the closed spaces formed by the linear segments (in the presentembodiment, the closed spaces formed by connecting the “V” and “M”shapes, as shown in FIG. 7), enabling a higher expansion holding forceto be achieved.

Furthermore, in the stent 301 of the present embodiment, when the stent301 is in the contracted state shown in FIG. 8, the various elements arearranged with substantially no intervening gaps in the circumferentialdirection. As a result, the stent 301 has a high coverage.

The stent has the shape, for example, as described above.

In cases where the stent has a shape like that described above and thebody is made of the ferrous alloy of the invention, even after placementof the stent in vasculature subject to large deformation, such as in thelegs, the stent does not fail due to large deformation and moreover hasan excellent durability (fatigue strength).

Although the stent will have a size which varies depending on the sitewhere it is to be placed, the outer diameter of the stent when expanded(i.e., when restored to the original shape and no longer contracted) ispreferably from 2.0 to 30 mm, and more preferably from 2.5 to 20 mm. Thestent length is preferably from 10 to 150 mm, and more preferably from15 to 100 mm. In particular, a stent according to the embodiment that isintended for placement within a blood vessel will have an outer diameterof preferably from 2.0 to 14 mm, and more preferably from 2.5 to 12 mm,and a length of preferably from 5 to 100 mm, and more preferably from 10to 80 mm.

The stent may have a wall thickness that is smaller than in conventionalstents. The body of the stent, because it is formed of the subsequentlydescribed ferrous alloy of the invention, has a high strength anddurability. Accordingly, even at a small wall thickness, the stent willpossess the desired strength and durability. For example, the wallthickness may be set to 0.2 mm or less, or even 0.10 mm or less.

The body of the stent may be fabricated by using a pipe made of theferrous alloy of the invention and removing (such as by cutting ormelting away) those portions of the pipe that do not form a part of thestent. In this way, an integral structure is obtained. A pipe used toform the body of the stent can be manufactured by melting the ferrousalloy of the invention in an inert gas or evacuated atmosphere to form aferrous alloy ingot, mechanically grinding the ingot, then hot pressingand extruding to form a large-diameter pipe. Successive die drawingsteps and heat treatment steps are then carried out repeatedly to reducethe pipe to a given wall thickness and outer diameter, after which thesurface of the resulting pipe is chemically or physically polished. Thestent base material can be formed from the pipe by a suitable processsuch as cutting (e.g., mechanical grinding, laser cutting), electricaldischarge machining, chemical etching, or a combination of suchtechniques.

The body of the stent is preferably made of braided wire, because anindwelling stent made of braided wire readily conforms to bodilymovement and vascular pulsation.

After it has been fashioned to the final shape of the stent, the body ofthe stent is preferably subjected to solution treatment at from 900 to1400° C., rapidly cooled at a rate of at least 50° C./s, then aged at atemperature of at least 200° C. but less than 800° C. By carrying outsuch solution treatment and aging treatment, the superelasticity andstrength of the stent body are improved, thereby enhancing theflexibility of the entire stent body, resulting in good ease ofplacement within tortuous blood vessels.

The diameter of the stent when unexpanded is preferably from about 0.8mm to about 1.8 mm, and more preferably from about 0.9 mm to about 1.6mm. The length of the stent when unexpanded is preferably from about 10mm to about 200 mm. The length of a single undulating ring is preferablyfrom about 8 mm to about 40 mm.

Although stents are generally provided with imaging markers, the stentsaccording to the present embodiment need not be provided with suchmarkers because, as noted above, the ferrous alloys of the inventionused in the stents have excellent fluoroscopic visualization properties.

Next, the ferrous alloys of the invention are described.

1. Crystal Structure and Properties of Ferrous Alloy

The ferrous alloys of the invention have what is substantially adual-phase structure composed of a γ phase having a face-centered cubic(fcc) structure serving as the matrix within which is finely dispersed aγ′ ordered phase having a L1₂ structure. Cooling the γ phase induces amartensitic transformation to an α′ phase having a body-centered cubic(bcc) structure, and heating once more brings about a reversetransformation to the matrix γ phase. The martensitic transformationstart temperature (Ms point) and the reverse transformation finishtemperature (Af point) can be determined by electrical resistancemeasurement. As shown in FIG. 10, in shape memory alloys, there isgenerally a hysteresis between the martensitic transformation and thereverse transformation.

Superelasticity in shape-memory alloys arises due to stress-inducedmartensitic transformations at and above the Af point and to the reversetransformations. However, if the hysteresis width is large, a highstress is required to induce the martensitic transformation, which tendsto result in the introduction of permanent strain such as dislocations,preventing a good superelasticity from being achieved. Therefore, bymaking the hysteresis width smaller, a martensitic transformation can beinduced at a lower stress, enabling good superelasticity to be achievedwithout the introduction of permanent strain such as dislocations at thetime of deformation. As a result of extensive research, the inventorshave found that the width of the thermal hysteresis for the ferrousalloys of the invention must be set to 100° C. or below to achieve suchsuperelasticity. The width of the thermal hysteresis is preferably 70°C. or less.

It is preferable for the ferrous alloys of the invention to have arecrystallization texture in which the specific crystal orientation<100> or <110> of the matrix γ phase is aligned with the cold workingdirection (e.g., rolling, wire drawing). The ferrous alloys of theinvention are able to achieve shape memory and superelastic propertieseven if the crystal orientations are completely random, although byhaving the foregoing specific crystal orientations aligned with theworking direction, even better shape memory and superelastic propertiesmay be achieved. Crystal orientations in the alloy texture can bemeasured by the electron backscatter diffraction pattern method and areexpressed herein as the “abundance,” which indicates the degree ofalignment by the crystal orientation. The abundance of the crystalorientation <100> in the cold-working direction is a ratio based on anarbitrary value of “1” when the crystal orientation is completelyrandom. A higher value indicates greater alignment of the crystalorientation.

The abundance of a specific crystal orientation in the working directionfor the ferrous alloys of the invention is preferably at least 2.0, andmore preferably at least 2.5.

The ferrous alloys of the invention which have a thermal hysteresis of100° C. or less and wherein, moreover, the crystal orientations in thematrix γ phase are aligned have a higher Young's modulus and strengthand a larger superelastic strain than Ti—Ni alloys. The Young's modulusis at least about 40 GPa, the 0.2% yield strength is at least about 600MPa, and the superelastic strain is at least 5%. In addition, theferrous shape-memory alloys of the invention are endowed with a goodhardness, tensile strength and elongation at break, and thus have anexcellent workability.

2. Composition of Ferrous Alloys (a) Basic Composition

The ferrous alloys of the invention have a basic composition whichincludes from 25 to 35% by mass of nickel, from 10 to 30% by mass ofcobalt, and from 2 to 8% by mass of aluminum. In addition, the basiccomposition also includes a total of from 1 to 20% by mass of at leastone selected from the group consisting of from 1 to 5% by mass oftitanium, from 2 to 10% by mass of niobium, and from 3 to 20% by mass oftantalum. The balance is iron and inadvertent impurities. In theexplanation provided herein of the ferrous alloy of the invention,unless noted otherwise, the contents of the respective elements arebased on the overall alloy (100% by mass).

The ferrous alloy of the invention includes preferably at least 30% bymass and more preferably at least 35% by mass of iron. The ferrous alloyincludes preferably not more than 55% by mass and more preferably notmore than 50% by mass of iron. Iron is preferably contained in a rangeof from 30 to 55% by mass, and more preferably from 35 to 50% by mass. Atoo low content of iron in the ferrous alloy of the invention tends tolower the cold workability and further toughness following the treatmentfor imparting superelasticity (aging treatment). On the other hand, atoo high iron content tends to increase the transformation hysteresis infcc to bcc martensitic transformation (to have a temperature of 100° C.or higher), thus making it impossible to achieve good superelasticity.

Nickel is an element which induces a martensitic transformation and alsolowers the temperature thereof. The ferrous alloy of the inventionpreferably includes from 25 to 35% by mass of nickel. By having a nickelcontent within this range, the martensitic transformation temperature ofthe ferrous alloy decreases, stabilizing the matrix phase (fcc phase).At a nickel content of more than 35% by mass, an excessive declineoccurs in the martensitic transformation temperature and transformationdoes not arise within a practical temperature range, as a result ofwhich a good shape memory effect and good superelasticity cannot beachieved.

In addition, nickel is an element which, in aging treatment, causes theprecipitation of ordered phases having an fcc and/or fct structure, suchas Ni₃Al. Such ordered phases strengthen the matrix phase of the ferrousalloy, in addition to which they reduce the thermal hysteresis ofmartensite, thereby enhancing the shape memory properties and thesuperelasticity. When the nickel content is less than 25% by mass, theamount of ordered phase that precipitates is insufficient, as a resultof which good shape memory properties and superelasticity are notachieved. The nickel content is more preferably from 26 to 30% by mass.

Cobalt is an element which lowers the modulus of rigidity of the matrixphase and reduces transformation strain, thus improving the shape memoryproperties. It is preferable for the inventive ferrous alloy to includefrom 10 to 30% by mass of cobalt. At a cobalt content above 30% by mass,the cold workability of the alloy may decrease. At a cobalt contentbelow 10% by mass, the above-indicated effects of cobalt added may notbe fully achieved. The cobalt content is more preferably from 15 to 23%by mass.

Aluminum is an element which, as with nickel, induces the precipitationof fcc and/or fct γ′ ordered phases of Ni₃Al or the like in agingtreatment. At an aluminum content below 2% by mass, the amount ofordered phase that precipitates is inadequate, thus making it impossibleto achieve good shape memory properties and superelasticity. On theother hand, at an aluminum content above 8% by mass, the ferrous alloybecomes very brittle. The ferrous alloy of the invention has an aluminumcontent of preferably from 2 to 8% by mass, and more preferably from 4to 6% by mass.

It is preferable for the ferrous alloy to additionally include a totalof from 1 to 20% by mass of at least one primary added element selectedfrom the group consisting of from 1 to 5% by mass of titanium, from 2 to10% by mass of niobium, and from 3 to 20% by mass of tantalum. Byincluding a primary added element, the amount of precipitation of the γ′ordered phase greatly increases, along with which the strength of thematrix phase rises considerably and the thermal hysteresis of martensitedecreases markedly, resulting in improvements in the shape memoryproperties and the superelasticity. At a total content of these elementsin excess of 20% by mass, the cold workability of the alloy maydecrease.

(b) Other Elements

The ferrous alloys of the invention may also include at least onesecondary added element selected from the group consisting of boron,carbon, calcium, magnesium, phosphorus, zirconium, ruthenium, lanthanum,hafnium, lead and misch metal. The total content of secondary addedelements is preferably 1% by mass or less, more preferably from 0.001 to1% by mass, and most preferably from 0.002 to 0.7% by mass. Thesecondary added elements suppress grain boundary reactions of β phasehaving a B2 structure which arise during aging, thereby improving theshape memory properties and the superelasticity.

The ferrous alloy of the invention may also include at least onetertiary added element selected from the group consisting of beryllium,silicon, germanium, manganese, chromium, vanadium, molybdenum, tungsten,copper, silver, gold, gallium, platinum, palladium, and rhenium. Thetotal content of tertiary added elements is preferably 10% by mass orless, more preferably from 0.001 to 10% by mass, and most preferablyfrom 0.01 to 8% by mass.

Of the tertiary added elements, silicon, germanium, vanadium,molybdenum, tungsten, gallium and rhenium increase coherence between thematrix γ phase and the γ′ ordered phase, enhances precipitationstrengthening of the γ′ phase, and improves the shape memory properties.The total content of these elements is preferably 10% by mass or less.

Beryllium and copper improve the strength of the matrix γ phase due tosolid solution strengthening, enhancing the shape memory properties. Thecontents of beryllium and copper are each preferably 1% by mass or less.

Chromium is an element which is effective for maintaining the wearresistance and corrosion resistance. The content of chromium ispreferably 10% by mass or less.

Manganese lowers the Ms point, thus making it possible to lower thecontent of costly nickel. The content of manganese is preferably 5% bymass or less.

Silver, gold, platinum and palladium have the effect of increasing thetetragonal character of α′ martensite, thus reducing the thermalhysteresis and improving the shape memory properties and thesuperelasticity. The content of these elements is preferably 10% by massor less.

3. Method of Producing the Ferrous Alloy (a) Cold Working

The inventive ferrous alloy having the above composition is formed intothe desired shape by melt casting, hot working and cold working.Solution treatment and aging are carried out following the shapingoperations. The shaping operation carried out prior to solutiontreatment is preferably a cold working process such as cold rolling,cold wire drawing or die pressing. Cold working is preferred because,following solution treatment, there can be obtained a recrystallizationtexture in which specific crystal orientations of the γ phase arealigned in the cold working direction. A larger superelastic strain canbe obtained in plates, pipes, wire and other worked materials havingsuch a texture than in materials having random orientations. Moreover,after cold working, surface working such as shot peening may be carriedif necessary. Hence, cold working enables plate, pipe, wire and otherworked materials to be obtained in which specific crystal orientationsof the γ phase are aligned with the working direction.

Because the reduction ratio (also referred to herein as the “workingratio”) that can be achieved in the ferrous alloy by a single coldworking pass is only about 10%, to achieve a high total reduction ratioin cold working, it is necessary to carry out a number of cold workingpasses. Cold working at this time may be carried out while interspersinga plurality of annealing treatment operations, in which case making thetotal reduction ratio after the final annealing operation as high aspossible is desirable for increasing the orientation of the alloytexture. Annealing treatment preferably involves heating at atemperature of from 900 to 1300° C. for a period of from 1 minute to 3hours. Cooling after annealing is preferably carried out by air cooling,and more preferably carried out by water cooling.

In the ferrous alloy of the invention, by means of cold working, the<100> or <110> orientation of the γ phase following solution treatmentcan be aligned with the cold working (e.g., rolling or wire drawing)direction. Crystal orientations in the alloy texture can be measured bythe electron backscatter diffraction pattern method, and the resultsused to determine the “abundance,” which indicates the degree ofalignment of the crystal orientations. For example, the abundance of<100> in the working direction is a ratio based on an arbitrary value of“1” for the theoretical case in which the crystal orientations arecompletely random. A higher abundance value indicates greater uniformityof the crystal orientations.

As a result of extensive investigations, the inventors have found thatwhen the abundance of a specific crystal orientation, such as <100> or<110>, of the γ phase is 2.0 or more, ferrous alloys having excellentshape memory properties and superelasticity can be obtained. In theferrous alloys of the invention, the abundance of the above specificcrystal orientation can be set by means of the total reduction ratioafter the final annealing operation. To increase the abundance of theabove specific crystal orientation, it is preferable for the totalreduction ratio following the final annealing operation to be as high aspossible. For this value to be 2.0 or more, regardless of the alloycomposition, the total reduction ratio in cold working after the finalannealing operation must be set to at least 50%. If the total reductionratio in cold working after the final annealing operation is low, thespecific crystal orientations in the alloy texture will not align withthe working direction, making it impossible to achieve sufficientimprovements in the shape memory properties and the superelasticity. Thetotal reduction ratio in cold working is preferably at least 70%, andmore preferably at least 92%.

(b) Solution Treatment

The cold-worked ferrous alloy is heated to the solid solutiontemperature and the crystal structure is transformed to an austenitic γphase (single phase), after which it is preferable to carry out solutiontreatment involving rapid cooling. Solution treatment is carried out ata temperature of at least 800° C. The solution temperature is preferablyfrom 900 to 1400° C. The holding time at the treatment temperature ispreferably from 1 minute to 50 hours. At less than 1 minute, asufficient solution treatment effect cannot be achieved, whereas at morethan 50 hours, the influence of oxidation becomes impossible todisregard.

Solution treatment may be carried out while applying a stress. Bycarrying out such “tension annealing,” the memory shape of the ferrousalloy can be precisely controlled. When stress is applied duringsolution treatment, the stress is preferably from 0.1 to 50 kgf/mm².

Following heat treatment, the γ single phase state can be frozen byquenching at a rate of at least 50° C./s. Quenching may be carried outby placing the ferrous alloy in a coolant such as water or by forced aircooling. When the cooling rate is set to less than 50° C./s, β phase (βphase having a B2 structure) precipitation occurs, as a result of whichshape memory properties may not be achieved. The cooling rate ispreferably at least 50° C./s.

(c) Aging Treatment

Following solution treatment, it is desirable to carry out agingtreatment. By carrying out aging treatment, an ordered phase of Ni₃Al orthe like having a fcc and/or fct structure appears, strengthening thematrix phase and reducing the thermal hysteresis of martensitictransformation, thereby improving the shape memory properties andsuperelasticity. Aging treatment is carried out at a temperature of atleast 200° C. but less than 800° C. Treatment at less than 200° C.results in insufficient precipitation of the above-mentioned orderedphase. On the other hand, treatment at 800° C. or above is undesirablebecause precipitation of the stable β phase occurs.

The aging treatment time varies depending on the ferrous alloycomposition and the treatment temperature. When treatment is carried outat a temperature of at least 700° C. but less than 800° C., the agingtreatment time is preferably in a range of from 10 minutes to 50 hours.On the other hand, when treatment is carried out at a temperature of atleast 200° C. but less than 700° C., the aging treatment time ispreferably in a range of from 30 minutes to 200 hours. When the agingtreatment time is shorter than the above-indicated time, the resultingeffects are inadequate. On the other hand, when the aging treatment timeexceeds the above-indicated time, β phase precipitation may occur,resulting in a loss of the shape memory properties.

EXAMPLES

The ferrous alloys of the invention are illustrated more fully below byway of examples, although the invention is not limited by the examples.

Examples 1 to 5, and Comparative Example 1

The ferrous alloys of Examples 1 to 5 and Comparative Example 1, whichare alike with the exception of the alloy compositions and the agingtreatment times, were produced by the method described below.

In the respective examples, an alloy having the composition shown inTable 1 below was melted, then solidified at an average cooling rate of140° C./min to form a billet having a diameter of 12 mm. The billet washot-rolled at 1300° C., giving a 1.3 mm thick plate. The hot-rolledplate was then subjected to a first annealing operation at 1300° C. for10 minutes, after which cold rolling was carried out several times to aplate thickness of 0.65 mm. A second annealing operation was thencarried out under the same conditions, then cold rolling was carried outseveral times to give a 0.2 mm thick plate. The total reduction ratioafter second annealing (final annealing) was 70%. The plate in eachexample was subsequently heat-treated at 1300° C. for 30 minutes, thenplunged into ice water and thereby quenched (solution treatment). Next,aging treatment at 600° C. was carried out, thereby giving a ferrousalloy plate which was composed of two phases—a γ phase having an fccstructure and a γ′ phase having a L1₂ structure, and which had shapememory properties and superelasticity. The production process, from thefirst annealing operation described above to aging treatment, isschematically shown in FIG. 11A. The aging treatment times for eachalloy are shown in Table 1.

Examples 6 to 9

The ferrous alloys of Examples 6 to 9 are alloys which have the samecomposition but for which the process conditions from the annealing tothe aging treatment operations differ. For example, the ferrous alloy ofExample 6 was produced by the following method.

The alloy having the composition shown in Table 1 below was melted, thensolidified at an average cooling rate of 140° C./min to form a billethaving a diameter of 20 mm. The billet was hot-rolled at 1300° C.,giving a 1.6 mm thick plate. The hot-rolled plate was then subjected toa first annealing operation at 1300° C. for 10 minutes, after which coldrolling was carried out several times to a plate thickness of 0.8 mm.Next, the following operations were carried out under the sameconditions: second annealing→cold rolling→third annealing→cold rolling,yielding a 0.2 mm thick plate. The total reduction ratio after thirdannealing (final annealing) was 50%. The resulting plate wassubsequently heat-treated at 1300° C. for 30 minutes, then plunged intoice water and thereby quenched (solution treatment). Next, agingtreatment at 600° C. was carried out for 90 hours, thereby giving aferrous alloy plate which was composed of two phases—a γ phase having anfcc structure and a γ′ phase having a L1₂ structure, and which had shapememory properties and superelasticity. The process for producing thealloy of Example 6, from the first annealing operation described aboveto aging treatment, is schematically shown in FIG. 11B.

The ferrous alloys in Examples γ to 9 were produced by changing thesequence of annealing and cold rolling operations used to produce theferrous alloy of Example 6 in the manner shown in FIGS. 11C to 11E. FIG.11C relates to Example 7, FIG. 11D relates to Example 8, and FIG. 11Erelates to Example 9. The total cold working ratios after the finalannealing operation are shown in Table 1.

TABLE 1 Cold Difference working Aging 0.2% between Af Abundance ratioafter treat- Young's Yield Super- point and of <100> in Ex. Other finalan- ment modulus strength elastic Ms point rolling No Fe Ni Co Al Ti NbTa elements nealing (%) time (h) (Ga) (MPa) strain (° C.) direction EX 146.4 30.7 14.9 5.8 2.2 70 48 76 630 Acceptable 67 2.6 EX 2 45.5 30.014.6 5.7 4.2 70 60 72 580 Acceptable 41 2.6 EX 3 43.6 28.9 14.0 5.5 8.070 60 65 300 Acceptable 31 2.5 EX 4 38.8 27.7 17.2 5.3 7.8 W: 3.2 70 7278 520 Acceptable 36 2.6 EX 5 40.2 28.8 17.6 5.4 8.0 B: 0.006 70 90 67765 Good 30 2.5 EX 6 40.2 28.8 17.6 5.4 8.0 B: 0.01 50 90 52 788Acceptable 32 2.3 EX 7 40.2 28.8 17.6 5.4 8.0 B: 0.01 75 90 51 772 Good30 2.8 EX 8 40.2 28.8 17.6 5.4 8.0 B: 0.01 90 90 50 760 Good 31 6.4 EX 940.2 28.8 17.6 5.4 8.0 B: 0.01 98 90 51 748 Excellent 32 11 CE 1 49.534.0 10.0 6.5 70 13 31 400 No Good 200 2.6 CE 2 50.0 50.0 — — 31 536Good — —

The temperature width of the thermal hysteresis of martensitictransformation and reverse transformation (temperature width=differencebetween Af point (reverse transformation finish temperature) and the Mspoint (martensitic transformation start temperature)), the abundance ofthe crystal orientation <100> in the rolling direction, the superelasticstrain (superelasticity), the Young's modulus, and the 0.2% yieldstrength for Examples 1 to 9 and Comparative Example 1 were measured bythe following methods. The results are shown in Table 1.

(1) Temperature Width of Thermal Hysteresis (Difference Between Af Pointand Ms Point)

The Ms point and the Af point for the plate were determined by electricresistance measurement (see FIG. 10), and the difference therebetweenwas treated as the temperature width of the thermal hysteresis.Measurement results for Examples 1 to 9 and Comparative Example 1 areshown in Table 1.

(2) Abundance of <100> in Rolling Direction

The abundance, in the rolling direction of the plate obtained, of aspecific crystal orientation in the γ phase was measured using anelectron backscatter diffraction pattern measuring apparatus(manufactured by TexSEM Laboratories, Inc. (TSL) under the trade nameOrientation Imaging Microscope). Measurement results for Examples 1 to 9and Comparative Example 1 are shown in Table 1.

The abundance of the crystal orientation was determined by usinganalysis software (Orientation Imaging Microscope Software Version 3.0available from TSL) following measurement of the crystal orientation bythe electron backscatter diffraction (EBSD) pattern method using theelectron backscatter diffraction pattern measuring apparatus. Morespecifically, the abundance was calculated by an analytical processusing a harmonic function incorporated in the analysis software with theexpansion order being 16 and the half-width when applied to a Gaussiandistribution being 5 degrees. The case where the crystal orientation wasnot aligned at all with the working direction was regarded as “0”, thecase where the crystal orientation was random as “1” and the case wherea larger abundance value of <100> in the working direction indicates,the crystal orientation is more aligned in a specific direction.

(3) Superelastic Strain (Superelasticity)

The superelastic strain was determined from stress-strain curvesobtained by a tension cycling test on the plate at room temperature.Typical measurement results are shown in FIG. 12. In the tension cyclingtest, a single cycle consisted of applying a fixed strain to the initialspecimen length and subsequently removing the load. The applied strainstarted at 2% in Cycle 1 and was increased by 2% in each successivecycle (to 4% in Cycle 2, and 6% in Cycle 3), the process being repeateduntil the specimen failed. The superelastic strain (ε_(SE) ^(i)) for thei^(th) cycle was determined, as shown in FIG. 12, by the followingformula from the stress-strain curve obtained for that particular cycle.

ε_(SE) ^(i)(%)=ε_(t) ^(i)−ε_(r) ^(i)−ε_(e) ^(i)

In the formula, i is the cycle number, ε_(t) ^(i) is the strain appliedin the i^(th) cycle, ε_(r) ^(i) is the residual strain in the i^(th)cycle, and ε_(e) ^(i) is the pure elastic deformation strain in thei^(th) cycle.

The maximum superelastic strain achievable up until failure of the platewas rated according to the following criteria. FIG. 13 shows thestress-strain curve when the maximum strain on the plate in Example 3was 2%.

-   Maximum superelastic strain was 8% or more:

Excellent

-   Maximum superelastic strain was at least 2% but less than 8%: Good-   Maximum superelastic strain was at least 0.5% but less than 2%:    Acceptable-   Maximum superelastic strain was less than 0.5%:

No Good

Measurement results obtained for Examples 1 to 9 and Comparative Example1 are shown in Table 1.

(4) Young's Modulus and 0.2% Yield Strength

As shown in FIG. 12, the Young's modulus and the 0.2% yield strengthwere measured from the stress-strain correlation diagram obtained intension tests at room temperature. Measurement results obtained forExamples 1 to 9 and Comparative Example 1 are shown in Table 1.

As is apparent from Table 1, each of Examples 1 to 9, wherein thetemperature width of the thermal hysteresis between the martensitictransformation and the reverse transformation was up to 100° C.,exhibited a superelasticity having a maximum superelastic strain of atleast 0.5%. However, in Comparative Example 1, wherein the thermalhysteresis temperature width was 200° C., the superelasticity was lessthan 0.5%. These results show that the ferrous alloys in Examples 1 to 9for which the temperature width of the thermal hysteresis was small havea better superelasticity than the ferrous alloy in Comparative Example 1for which the temperature width of the thermal hysteresis was large.

The ferrous alloys in Examples 6 to 9 are alloys of the same compositionthat were produced under different conditions in the annealing to agingtreatment operations. These alloys each have recrystallization textureswith differing degrees of alignment for specific crystal orientations inthe γ phase. FIGS. 14A and 14B show inverse pole figures which indicateas contour lines the abundance of various crystal orientations in therolling direction of the plate obtained in Example 6 (FIG. 14A) and theplate obtained in Example 9 (FIG. 14B). In FIG. 14A of the plateobtained in Example 6, the contour lines are clustered in the <100>direction, indicating that the <100> direction is aligned with therolling direction. The abundance ratio of <100> in the rolling directionwas 2.3. In FIG. 14B of the plate obtained in Example 9, the abundanceratio of <100> in the rolling direction was 11.0, indicating that the<100> orientation was even more strongly aligned with the rollingdirection. Hence, in the ferrous alloys of the invention, the larger thetotal cold working ratio after final annealing, the more strongly thespecific crystal orientations of the γ phase are aligned with therolling direction.

FIG. 15 is the stress-strain curve when the maximum strain in Example 9is 15%. It is evident here that a superelastic strain of about 10% canbe obtained.

As is apparent from Table 1, ferrous alloys in which the total coldworking ratio after final annealing is higher and the specific crystalorientations are more strongly aligned have a larger superelasticstrain.

Table 1 also shows the superelasticity, Young's modulus and yieldstrength of a Ti—Ni alloy as Comparative Example 2. It is apparent thatthe inventive ferrous alloys which have a small thermal hysteresis andin which specific crystal orientations are strongly aligned are endowedwith a larger superelastic strain and a higher Young's modulus andstrength than the Ti—Ni alloy.

Guide wires and stents including at least a member made of the ferrousalloy of the invention have been described above, although the inventiveferrous alloys may also be used as a chief or partial material in othermedical devices used by being inserted or implanted into the body andother medical devices used outside the body. Examples of such othermedical devices include vascular filters, orthodontic wire, artificialtooth roots, catheters, bone plates, intramedullary pins, staples,cerebral artery clips, vasooclusive devices and medical forceps.

FIG. 16 is a perspective view of a vascular filter composed of theferrous alloy of the invention. The vascular filter 410 shown in FIG. 16is a device which is designed so as to be intravascularly advanced tothe inferior vena cava, for example, where it is expanded and functionsto prevent thrombus migration from the lower part of the body to theheart and lungs. It generally has a bell-like shape. The vascular filter410 has an outwardly flaring region 417 and has a convergent zone 421 ina filtration region 419. The filter 410 has a size in the transversedirection at the flaring (or attachment/securing) region 417 which islarger than that at the filtration region 419. In the outwardly flaringregion 417, elongated struts 414 having open intervals as shown in FIG.16 extend out at some fixed angle from the longitudinal axis of thevascular filter 410. In the filtration region 419 which begins at anintermediate portion (the place of transition between the outwardlyflaring region 417 and the filtration region 419) of the filter, thestruts 414 curve or bend inward (region 423) toward the longitudinalaxis, then extend inward at some fixed angle with respect to a tubularportion 418, thereby forming some fixed angle with the longitudinalaxis.

At the flaring or attachment (securing) region 417, each strut 414divides into two joined strut portions 414 a and 414 b. Strut portions414 a and 414 b of each divided strut 414 include curved regions 425which extend in opposite directions toward corresponding strut portions414 a and 414 b of the respectively neighboring struts. The joined strutportion 414 a on one strut and a separate strut portion 414 b convergeat an end 429 of the filter to form a substantially V-shaped region.

The vascular filter composed of the ferrous alloy of the invention mayserve as an element of a thrombus capturing device which has a shaft toenable insertion into and removal from a vascular lumen, and which canoperate the filter between an expanded state and a contracted state.

The blood filter made of the ferrous alloy of the invention hasexcellent fluoroscopic visualization properties, making it easy todetermine the placement position. Moreover, it is adaptable to variousvascular diameters and, in the contracted state, is capable of achievinga very narrow outer diameter, making it easy to insert into a desiredblood vessel.

FIG. 17 is a perspective view of an orthodontic wire composed of theferrous alloy of the invention. The orthodontic wire 520 shown in FIG.17 has a curved portion 522 which is substantially U-shaped in a topview. The orthodontic wire 520 has a cross-sectional shape that issubstantially square, but may instead be round. Because the Young'smodulus and 0.2% yield strength are larger than those for conventionalTi—Ni alloys, the same corrective force is achieved with a smallercross-section, thus making it possible to use smaller brackets andreducing patient discomfort when worn. Moreover, because the orthodonticwire 520 is made of the above-described ferrous alloy and has a largersuperelasticity, the number of adjustments made to the wire can bereduced.

1. A stent comprising a stent body made of a ferrous alloy which hasshape memory properties and superelasticity, includes substantially twophases having a γ phase and a γ′ phase, and has a difference of 100° C.or less between a reverse transformation finish temperature and amartensitic transformation start temperature in a thermal hysteresis ofmartensitic transformation and reverse transformation, the ferrous alloyincluding from 30 to 50% by mass of iron; wherein the stent isconfigured for insertion into a living body.
 2. The stent of claim 1,wherein the stent body includes a plurality of undulating bends providedin an axial direction.
 3. The stent of claim 1, wherein the stent bodyis composed of braided wire.
 4. The stent of claim 1, wherein the stentbody has a wall thickness of 0.2 mm or less.
 5. The stent of claim 4,wherein the stent body has a wall thickness of 0.10 mm or less.
 6. Thestent of claim 1, wherein the ferrous alloy includes from 38.8 to 46.4%by mass of iron.
 7. The stent of claim 1, wherein an abundance of aspecific crystal orientation in a working direction of a worked portionfor the ferrous alloy is at least 2.0.
 8. A stent comprising a stentbody comprising a ferrous alloy having shape memory properties andsuperelasticity, including two phases having a γ phase and a γ′ phase,and having a difference of 100° C. or less between a reversetransformation finish temperature and a martensitic transformation starttemperature in a thermal hysteresis of martensitic transformation andreverse transformation; the ferrous alloy comprising a composition whichincludes from 25 to 35% by mass of nickel, from 10 to 30% by mass ofcobalt, and from 2 to 8% by mass of aluminum, the composition includinga total of from 1 to 20% by mass of at least one selected from the groupconsisting of from 1 to 5% by mass of titanium, from 2 to 10% by mass ofniobium, from 3 to 20% by mass of tantalum, and the balance being from35 to 50% by mass of iron and inadvertent impurities; and wherein thestent being configured for insertion into a living body.
 9. The stent ofclaim 8, wherein an abundance of a specific crystal orientation in aworking direction of a worked portion for the ferrous alloy is at least2.0.
 10. The stent of claim 8, wherein the ferrous alloy includes from38.8 to 46.4% by mass of iron.
 11. The stent of claim 8, wherein thestent body includes a plurality of undulating bends provided in an axialdirection.
 12. The stent of claim 8, wherein the stent body is composedof braided wire.
 13. The stent of claim 8, wherein the stent body has awall thickness of 0.2 mm or less.
 14. The stent of claim 13, wherein thestent body has a wall thickness of 0.10 mm or less.
 15. The stent ofclaim 8, wherein the ferrous alloy comprises a composition whichincludes from 25 to 35% by mass of nickel, from 10 to 30% by mass ofcobalt, from 2 to 8% by mass of aluminum, from 3 to 20% by mass oftantalum, and the balance being from 35 to 50% by mass of iron andinadvertent impurities.
 16. A method of forming the stent of claim 1,comprising: providing a pipe made of the ferrous alloy; removingportions of the pipe to form a plurality of undulating rings.
 17. Themethod of claim 16, wherein said step of providing a pipe made of theferrous alloy further comprises: melting the ferrous alloy to form aferrous alloy ingot; mechanically grinding the ingot; hot pressing andextruding the ingot to form a large diameter pipe; heat treating anddrawing the large diameter pipe through a die to reduce the largediameter pipe to a predetermined wall thickness and a predeterminedouter diameter.